Technical Field
The present invention relates to a variable capacitance device and an antenna apparatus that uses the variable capacitance device.
Background Art
In mobile FeliCa near field communication (NFC) modules, there is a phenomenon that occurs in which variance in the coils of an antenna causes the resonant frequency of 13.56 MHz to shift, thereby deteriorating the receiving sensitivity of the modules, for example. Thus, a frequency-adjusting circuit that includes capacitors is incorporated into the modules, all devices are checked during shipment, and the capacitances of the capacitors are finely adjusted to correct the shift in the resonant frequency.
Conventionally, switched capacitors, in which field effect transistor (FET) switches are connected in series in a fixed capacitance element, have been used. A setting to switch the FETs is written in advance into a control integrated circuit (IC) when being checked for shipment to switch the FETs when the NFC is in use, thus finely adjusting the capacitances of the capacitors.
On the other hand, recently, there has been research in switching to general-purpose ceramic capacitors that have excellent breakdown voltage and are cheaper compared to the FET switches. Ceramic capacitor materials have a characteristic in which the capacitance decreases as DC bias voltage is applied, and it is this characteristic that is being proactively utilized.
There has also been research in adopting a variable capacitance device that uses a plurality of variable capacitance elements that include a dielectric layer formed using thin films as opposed to a sintered body, because ceramic capacitors have problems such as capacitance changing over time.
However, when a conventional variable capacitance device is inserted into an apparatus in the wrong direction, there is a possibility that sufficient capacitance variability cannot be obtained even when voltage is applied, because the variable capacitance device has directionality due to its structure.
FIGS. 1A and 1A show an example configuration of a conventional variable capacitance device, for example. In this conventional variable capacitance device, variable capacitance elements C101-C104 are connected in series between an input terminal IN and an output terminal OUT, and bias applying terminals X, Y are provided to the right and to the left of the variable capacitance elements C101-C104. As shown in FIG. 1A, a correct connection (also referred to as a forward connection) for this conventional variable capacitance device is one in which the terminal X, which is connected to the variable capacitance elements C101-C104 through three resistors, is connected to ground GND, and a prescribed voltage DC+ is applied to the terminal Y, which is connected to the variable capacitance elements C101-C104 through two resistors. The current flows from the terminal Y towards the terminal X in the directions shown by the arrows.
On the other hand, as shown in FIG. 1B, an incorrect connection (also referred to as a reverse connection) is one in which the terminal Y is connected to ground GND and a prescribed voltage DC+ is applied to the terminal X. In this case, the current flows from the terminal X towards the terminal Y in the directions shown by the arrows. The current does not flow to the variable capacitance elements C101, C104, and the applied voltage does not change.
As shown in FIG. 2, in the case of the forward connection, when DC+=0 V, the capacitance of each of the variable capacitance elements C101-C104 is 400 pF, and when DC+=+3 V, the capacitance of each of the variable capacitance elements C101-C104 decreases 33%, becoming 268 pF, for example. Thus, when DC+=0 V, the capacitance as a whole is 100 pF, and when DC+=+3 V, the capacitance becomes 67 pF, thereby changing the capacitance as a whole by 33%.
On the other hand, in the case of the reverse connection, when DC+=+3 V, the capacitances of the variable capacitance elements C102, C103 decrease by 33% to become 268 pF, but the capacitances of the variable capacitance elements C101, C104 do not change. Accordingly, when DC+=0 V, the capacitance as a whole is 100 pF, and even when DC+=+3 V, the capacitance becomes 80 pF, thereby changing the capacitance as a whole by only 20%.
Thus, it is not possible to sufficiently adjust the capacitances of the capacitors, creating a situation in which deviations in resonant frequency cannot be sufficiently corrected.